1. Field of the Invention
The present invention generally relates to an ink-jet printing apparatus, and more particularly, to an improved ink jet printing apparatus that can effectively prevent the viscosity of the ink filled in the ejecting nozzles from increasing and reduce ink jam at the ejecting nozzles.
2. Description of the Related Art
Recently, ink-jet printers that are usable as an output apparatus of an information processing apparatus (e.g., computers) have attracted a lot of attention. With an ink-jet printer, the running cost is less, while highly precise imaging and coloring processes are realized with a relatively simple technique. However, there are still many points to be improved in the ink-jet recording head of ink-jet printers.
The ink-jet head of an ink jet printer has a plurality of nozzles and pressure chambers communicating with the openings of the respective nozzles. An ejection energy generating means is driven in response to a print command to change the pressure in the chamber, which causes the ink to be ejected from the nozzles.
Ink droplets attached onto a recording medium may blur depending on the quality of the recording medium (e.g., paper quality), or may undesirably come into contact with other members and be smeared. To avoid such situations, ink is generally prepared so that the solvent promptly permeates the recording medium and then evaporates to fix the ink on the recording medium. However, this causes another problem, that is, the viscosity of ink increases due to evaporation of the solvent, especially in those nozzles that are not frequently used. The same problem also occurs it the printing operation is suspended for a while. Increase in ink viscosity causes ink jam. This problems becomes conspicuous when using an aqueous (water) ink containing a high-viscosity organic solvent, such as a humectant, containing as main components water and glycol or similar material. To prevent this problem, the nozzle openings are capped when the printing operation is suspended for a relatively long time, thereby preventing the evaporation of the ink solvent.
However, this countermeasure cannon prevent the ink jam from occurring during the printing operation in general, ink ejection is not uniform at all the nozzles, and there are some nozzles whose ejection frequency is quite low or which do not eject ink tar a long time, depending on the positional arrangement of the nozzle openings or on print data. At these nozzles, the ink solvent evaporates, and the viscosity of the ink filled in the nozzles increase. This results in the ink ejecting condition changing, and the dot positions are offset from the correct positions on the recording medium. In addition, ink ejection becomes unstable, and dots deform or are lacking. In the worst case, ink can not be ejected from the nozzles due to ink jam.
To overcome this problem, a technique for refreshing the ink in the nozzle to return the ink condition to the initial state has been proposed. This technique is used in a serial type printer in which the ink head moves with respect to the recording medium to print. With this technique, the recording head recedes to the non-printing area (i.e., the return area for the to-and-fro motion) after it carried out the printing operation for a predetermined time. In the non-printing area, a prescribed number of ink droplets are forcibly ejected from all the nozzle openings (which is referred to as spraying) to refresh the ink in the nozzle, and thereby returning to the initial condition.
However, this technique has several drawbacks. First, the printing operation has to be stopped regularly, and the printing rate is reduced. In addition, since in recent years the quantity of an ink droplet forming a dot is as little as several pico liters (pL), the interval between the spraying must be shortened to prevent the viscosity of the ink from increasing. This further causes the printing rate to decrease. Furthermore, the spray operation causes the ink consumption to increase. In order to conduct the spraying in a line printer in which a recording medium moves with respect to the recording head, an extra space for carrying out the spraying is needed, and the entire apparatus becomes large.
In conclusion, spraying cannot entirely solve the problems associated with increase of the ink viscosity.
There are some other known techniques for preventing ink jam. For example, Japanese Patent Application Laid-open (Kokai) No. 55-123476 discloses a printing apparatus using piezoelectric elements as ink-ejection energy generating means. In this apparatus, during the printing operation, a printing signal is applied, via a current-control resistance, to the piezoelectric element in the pressure chamber that corresponds to the nozzle not ejecting the ink, thereby slightly vibrating the liquid surface in the nozzle.
Japanese Patent Publication-after-Examination (Kokoku) No. 62-33074 and Japanese Patent Application Laid-open No. 4-80037 also disclose a technique for preventing ink jam by applying a voltage to the piezoelectric elements so as not to eject a ink droplet, but to vibrate the liquid surface at the nozzle openings. With this technique, it is not necessary for the apparatus to stop the printing operation, and therefore, the printing rate can be maintained, while preventing unnecessary ink consumption.
Slightly vibrating the liquid surface of the ink causes the ink particles to diffuse in the nozzle, thereby preventing the viscosity from increasing. This method is effective when using ink whose viscosity changes only gently and when the non-ejecting time is relatively short. If the non-ejecting time is long, the increase in viscosity can be still prevented by combining the liquid-surface vibrating method with the spraying technique.
However, in order to slightly vibrate the ink surface in the nozzle, a voltage is always applied to the ink ejecting means including during the non-printing period. This leads to still another problem in that the ink-ejection energy generating means wears easily, and the service life is shortened.
The present invention was conceived in view of the above-described problems, and it is an object of the present invention to provide an ink-jet printing apparatus that can produce slight vibration on the liquid surface in the nozzle in a efficient manner, while reducing the number of times of driving the ink-ejection energy generators. To achieve this object the ink-jet printing apparatus according to the present invention makes positive use of a so-called crosstalk phenomenon, in which the liquid surface of ink at a nozzle opening slightly vibrates under the influence of ink ejection from the adjacent nozzles.
In one aspect of the invention, an ink-jet printing apparatus comprises a plurality of nozzles configured to eject ink; pressure chambers, each corresponding to one of the nozzles; ink-ejection energy generators, each being provided to one of the pressure chambers; and a vibration controller configured to apply a non-ejecting vibration signal to the ink-ejection energy generator to cause the liquid surface of ink filled in the associated nozzle to slightly vibrate. The nozzles are grouped into two or more groups, and the vibration controller supplies the non-ejecting vibration signal to the ink-ejection energy generators of each nozzle group at a different timing. The voltage of the non-ejecting vibration signal is regulated so as not to cause the ink to eject from the nozzle, but only to slightly vibrate at the opening of each nozzle.
In other words, the vibration controller drives and causes the ink-ejection energy generator no produce energy that does not eject the ink from the nozzle, but causes the ink surface to slightly vibrate at the nozzle opening. The non-ejecting vibration signal is supplied sequentially to each nozzle group, varying the signal supply timing for each group. With this arrangement, the ink surface at a nozzle opening is always slightly vibrating, even when the non-ejecting vibration signal is not applied to this nozzle, because of propagation of vibration from the ink-ejection energy generators of the adjacent pressure chambers. The groups of ink-ejection energy generators take a rest group by group without application of non-ejecting vibration signal, while the ink surface of the associated nozzles are continuously slightly vibrating due to propagation of vibration from the adjacent nozzles. This arrangement allows the service life of the ink-ejection energy generators to be extended, while efficiently preventing the ink viscosity from increasing.
In this manner, the present invention positively makes use of the crosstalk, in which the vibration generated to eject ink for the printing operation affects the adjacent or neighborhood nozzles through structural members or through the ink channels. The reference frequency of the ink-ejection energy generator, which is used to cause the ink surface to slightly vibrate to prevent an increase in viscosity, is divided by a natural number to reduce the number of the ink-ejection energy generators driven at a time to half (xc2xd), one third (⅓), and so on. This arrangement can prevent the ink-ejection energy generator from deteriorating and extend their service life, while promoting the diffusion of ink inside the nozzle, Consequently, increase of ink viscosity and ink jam can be effectively prevented.
Conventionally, an effort has been made to reduce the crosstalk as much as possible in order to improve the particle property, which indicates the ink-ejection performance during printing. However it is difficult to completely remove the influence of the crosstalk because of the structure and the size of the recording head. For example, it is desirable for the variation in the ink-ejection rate due to crosstalk to be less than 20% and less than 10% is more desirable. However, it is difficult to make the variation less than 5%. If piezoelectric elements are used as ink-ejection energy generators, 10% to 20% variation in the ink-ejection rate corresponds to 5% to 15% variation in the driving voltage applied to the piezoelectric elements, although such a correspondence depends on various conditions, such as the structure of the ink head.
From the viewpoint of the vibration at the liquid surface utilized in the present invention, the vibration effect becomes smaller as crosstalk decreases. Accordingly, it is preferable to make use of crosstalk in the range driving every other nozzle or every few nozzles. This arrangement can maintain slight vibration at the liquid surface in all the nozzles, while preventing the ink viscosity from increasing. In addition, the number of times of driving the ink-ejection energy generator in each nozzle can be greatly reduced, and consequently, the service life of the ink-ejection energy generator can be extended.
The vibration controller used in the ink-jet printing apparatus according to the present invention has a reference frequency for slightly vibrating the ink surface, and it includes a frequency-dividing vibrating unit for driving the respective groups of ink-ejection energy generators, while shifting the phase for each nozzle group.
The ink-ejection energy generators are driven group by group by dividing the reference frequency by the number of nozzle groups. Accordingly, there is a rest period for each nozzle group, in which the ink-ejection energy generators belonging to that group are not driven. However, the liquid surface of the ink filled in the nozzles in the non-driven nozzle group can continuously vibrate at the reference frequency because of propagation of vibration from the adjacent ink-ejection energy generators belonging to different nozzle groups.
Preferably, the nozzles are grouped so that every predetermined number of nozzles belongs to the same nozzle group. For example, if the nozzles are grouped into three, then the nozzles are arranged according to a geometric rule, such as belong to the first group, the second group, the third group, the first group, the second group, the third group, and so on. When the ink-ejection energy generators in the first nozzle group are driven, the vibration propagates to the nozzles in the other groups, and consequently, the ink surface vibrates in all the nozzles, even if the ink-ejection energy generators in the other groups are in the non-driven period.
The ink-jet printer may have a printing area, in which ink is ejected from the nozzles to print data on the recording medium, and a non-printing area, in which the printing operation is not carried out the vibration controller applies the non-ejecting vibration signal to the ink-ejection energy generators when the nozzles are located in the non-printing area.
With this arrangement, the liquid surface of the ink filled in the nozzle slightly vibrates in the non-printing area, while vibration is not caused at the ink surface when the nozzle is located in the printing area. Accordingly, the printing operation is carried out in the desirable condition, while the ink viscosity is prevented from increasing.
The ink-jet printing apparatus may be of a serial type, which is designed so that the ink-jet recording head is moved (or scanned) with respect to the recording medium, and the vibration controller drives the ink-ejection energy generators for each nozzle group in the non-printing area when the recording head returns during its to-and-fro motion.
Since, with the serial-type ink-jet printing apparatus to which the present invention is applied, non-ejecting vibration (or slight vibration) is applied to the ink filled in the nozzles only in the non-printing area (or the return area), dot offset or ink jam can be prevented during the printing operation.
The ink-ejection energy generator is, for example, a piezoelectric element. Alternatively, a mechanism for boiling the ink by a heater and ejecting the ink by bubble pressure may be employed as the ink-ejection energy generator. Using a piezoelectric element is preferable because the piezoelectric element itself deforms in response to a driving voltage and the vibration effectively propagates to the surrounding members. The vibration generated by the ink-ejection energy generator causes the liquid surface (or the ink surface) to vibrate through the ink itself; however, allowing the vibration to propagate through a structural member of the recording head is more preferable.
From the above-described viewpoint, it is preferable that the pressure chamber is formed integrally with the ink-jet recording head in order to allow the vibration to propagate to the adjacent nozzles more effectively.
Preferably, the non-ejecting reference frequency of the vibration controller is synchronized with the frequency of the ink-ejection driving signal used in the printing operation. With this arrangement, the slight vibration generated in the non-printing area for the purpose of preventing the ink viscosity from increasing does not adversely affect the ink-ejecting operation during the printing.
The ink-jet printing apparatus may conduct a spraying operation in the non-printing area, in combination with the slight vibration applied to the ink surface. This arrangement can prevent the ink viscosity from increasing more efficiently.
The present invention is applicable to a so-called line printer having a highly integrated ink head with a number of nozzles, in which the recording medium is moved with respect to the high-integration ink head. In this case, the same effects and advantages can be achieved, namely, making use of the influence of the vibration from the adjacent ink-ejection energy generators to prevent ink-dot offset and ink jam, while extending the service life of the ink-ejection energy generator.